The present invention relates to a method and apparatus for making a ceramic or other articles to reduce dimensional changes as they cool from an elevated temperature to a lower temperature.
Most manufacturers of gas turbine engines are evaluating advanced investment cast turbine airfoils (i.e. turbine blade or vane) which include intricate air cooling channels to improve efficiency of airfoil internal cooling to permit greater engine thrust and provide satisfactory airfoil service life. Internal cooling passages are formed in the cast airfoils using one or more thin ceramic cores positioned in a ceramic shell mold where the molten metal is cast in the mold about the core. After the molten metal solidifies, the mold and core are removed to leave a cast airfoil with one or more internal passages where the cores formerly resided.
The ceramic core is typically made using a plasticized ceramic compound comprising ceramic flour, binder and various additives. The ceramic compound is injection molded, or transfer molded at elevated temperature in a core die or mold. When the green core is removed from the die or mold, it typically is placed between top and bottom rigid setters to cool to ambient temperature before core finishing and gauging operations. Since green ceramic cores can warp or twist within the gaps in the rigid setters during cooling, cores can exhibit dimensional variations from one core to the next in a production run of cores. Moreover, the green core may be improperly contacted by the top or bottom setter such that dimensional variations from one core to the next occur in a production run.
An object of the present invention is to provide a method of making a ceramic casting core and other articles using an improved constraint system that reduces dimensional variations as the core/article cools from an elevated temperature to a lower temperature and provides greater uniformity from green core finishing operations.
In one embodiment of the invention, method and apparatus are provided for making a ceramic core comprising constraining an as formed (green) ceramic core at elevated (superambient) temperature between a compliant member and a relatively rigid setter while the green core cools to a lower temperature. In one embodiment, a green ceramic core at elevated temperature is constrained between a pneumatic or other fluid pressurized compliant bladder and a rigid setter during cooling of the green core to ambient temperature. The compliant bladder may be communicated to a fluid pressurized cavity in a top setter to this end.
In another embodiment, a green ceramic core at elevated temperature is constrained between a first setter comprising a compliant, non-rigid material or facing layer, such as rubber, and a relatively rigid second setter during cooling of the green core to ambient temperature under an applied force.
The present invention is advantageous in making ceramic cores in that a green ceramic core is constrained during cooling to a lower temperature under applied force using a compliant member that can deform enough against the green core to increase surface contact therebetween, reduce localize regions of stress on the core, and promote greater contact of the core with the top and bottom setters to thereby reduce dimensional variations from one core to the next. Similar advantages are achieved in constraining other articles prone to deformation or distortion as they cool from an elevated forming (e.g. molding) or treating (e.g. heat treatment) temperature to a lower temperature. The present invention can be used to this end in manufacture of such other articles as ceramic articles other than ceramic cores, wax patterns used in investment casting, metal or other powder injection molded articles, plastic articles, and the like.